Expression of large tenascin-C splice variants in synovial fluid of patients with rheumatoid arthritis

Get Spliced: Uniting Alternative Splicing and Arthritis

Immune responses demand the rapid and precise regulation of gene protein expression. Splicing is a crucial step in this process; ~95% of protein-coding gene transcripts are spliced during mRNA maturation. Alternative splicing allows for distinct functional regulation, as it can affect transcript degradation and can lead to alternative functional protein isoforms. There is increasing evidence that splicing can directly regulate immune responses. For several genes, immune cells display dramatic changes in isoform-level transcript expression patterns upon activation. Recent advances in long-read RNA sequencing assays have enabled an unbiased and complete description of transcript isoform expression patterns. With an increasing amount of cell types and conditions that have been analyzed with such assays, thousands of novel transcript isoforms have been identified. Alternative splicing has been associated with autoimmune diseases, including arthritis. Here, GWASs revealed that SNPs associated with arthritis are enriched in splice sites. In this review, we will discuss how alternative splicing is involved in immune responses and how the dysregulation of alternative splicing can contribute to arthritis pathogenesis. In addition, we will discuss the therapeutic potential of modulating alternative splicing, which includes examples of spliceform-based biomarkers for disease severity or disease subtype, splicing manipulation using antisense oligonucleotides, and the targeting of specific immune-related spliceforms using antibodies.

Extracellular matrix complexity in biomarker studies: a novel assay detecting total serum tenascin-C reveals different distribution to isoform-specific assays

Serum biomarkers are the gold standard in non-invasive disease diagnosis and have tremendous potential as prognostic and theranostic tools for patient stratification. Circulating levels of extracellular matrix molecules are gaining traction as an easily accessible means to assess tissue pathology. However, matrix molecules are large, multimodular proteins that are subject to a vast array of post-transcriptional and post-translational modifications. These modifications often occur in a tissue- and/or disease-specific manner, generating hundreds of different variants, each with distinct biological roles. Whilst this complexity can offer unique insight into disease processes, it also has the potential to confound biomarker studies. Tenascin-C is a pro-inflammatory matrix protein expressed at low levels in most healthy tissues but elevated in, and associated with the pathogenesis of, a wide range of autoimmune diseases, fibrosis, and cancer. Analysis of circulating tenascin-C has been widely explored as a disease biomarker. Hundreds of different tenascin-C isoforms can be generated by alternative splicing, and this protein is also modified by glycosylation and citrullination. Current enzyme-linked immunosorbent assays (ELISA) are used to measure serum tenascin-C using antibodies, recognising sites within domains that are alternatively spliced. These studies, therefore, report only levels of specific isoforms that contain these domains, and studies on the detection of total tenascin-C are lacking. As such, circulating tenascin-C levels may be underestimated and/or biologically relevant isoforms overlooked. We developed a highly specific and sensitive ELISA measuring total tenascin-C down to 0.78ng/ml, using antibodies that recognise sites in constitutively expressed domains. In cohorts of people with different inflammatory and musculoskeletal diseases, levels of splice-specific tenascin-C variants were lower than and distributed differently from total tenascin-C. Neither total nor splice-specific tenascin-C levels correlated with the presence of autoantibodies to citrullinated tenascin-C in rheumatoid arthritis (RA) patients. Elevated tenascin-C was not restricted to any one disease and levels were heterogeneous amongst patients with the same disease. These data confirm that its upregulation is not disease-specific, instead suggest that different molecular endotypes or disease stages exist in which pathology is associated with, or independent of, tenascin-C. This immunoassay provides a novel tool for the detection of total tenascin-C that is critical for further biomarker studies. Differences between the distribution of tenascin-C variants and total tenascin-C have implications for the interpretation of studies using isoform-targeted assays. These data highlight the importance of assay design for the detection of multimodular matrix molecules and reveal that there is still much to learn about the intriguingly complex biological roles of distinct matrix proteoforms.

Tissue microenvironment dictates inflammation and disease activity in rheumatoid arthritis

The recent advance in treatments for rheumatoid arthritis (RA) has significantly improved the prognosis of RA patients. However, these novel therapies do not work well for all RA patients. The unmet need suggests that the current understanding about how inflammatory response arises and progresses in RA is limited. Recent accumulating evidence reveals an important role for the tissue microenvironment in the pathogenesis of RA. The synovium, the main tissue where the RA activity occurs, is composed by a unique extracellular matrix (ECM) and residing cells. The ECM molecules provide environmental signals that determine programmed site-specific cell behavior. Improved understanding of the tissue microenvironment, especially how the synovial architecture, ECM molecules, and site-specific cell behavior promote chronic inflammation and tissue destruction, will enhance deciphering the pathogenesis of RA. Moreover, in-depth analysis of tissue microenvironment will allow us to identify potential therapeutic targets. Research is now undertaken to explore potential candidates, both cellular and ECM molecules, to develop novel therapies. This article reviews recent advances in knowledge about how changes in cellular and ECM factors within the tissue microenvironment result in propagation of chronic inflammation in RA.

Biased TCR gene usage in citrullinated Tenascin C specific T-cells in rheumatoid arthritis

We aimed to search for common features in the autoreactive T cell receptor (TCR) repertoire in patients with rheumatoid arthritis (RA), focusing on the newly identified candidate antigen citrullinated Tenascin C (cit-TNC). Mononuclear cells from peripheral blood or synovial fluid of eight RA-patients positive for the RA-associated HLA-DRB1*04:01 allele were in-vitro cultured with recently identified citrullinated peptides from Tenascin C. Antigen-specific T cells were isolated using peptide-HLA tetramer staining and subsequently single-cell sequenced for paired alpha/beta TCR analyses by bioinformatic tools. TCRs were re-expressed for further studies of antigen-specificity and T cell responses. Autoreactive T cell lines could be grown out from both peripheral blood and synovial fluid. We demonstrate the feasibility of retrieving true autoreactive TCR sequences by validating antigen-specificity in T cell lines with re-expressed TCRs. One of the Tenascin C peptides, cit-TNC22, gave the most robust T cell responses including biased TCR gene usage patterns. The shared TCR-beta chain signature among the cit-TNC22-specific TCRs was evident in blood and synovial fluid of different patients. The identification of common elements in the autoreactive TCR repertoire gives promise to the possibility of both immune monitoring of the autoimmune components in RA and of future antigen- or TCR-targeted specific intervention in subsets of patients.

Elevated Serum Tenascin-C Predicts Mortality in Critically Ill Patients With Multiple Organ Dysfunction

Background: Multiple organ dysfunction is a complex and lethal clinical feature with heterogeneous causes and is usually characterized by tissue injury of multiple organs. Tenascin-C (TNC) is a matricellular protein that is rarely expressed in most of the adult tissues, but re-induced following injury. This study aimed to evaluate serum TNC in predicting mortality in critically ill patients with multiple organ dysfunction. Methods: Adult critically ill patients with at least two organs dysfunction and an increase of Sequential Organ Failure Assess (SOFA) score ≥ 2 points within 7 days were prospectively enrolled into two independent cohorts. The emergency (derivation) cohort was a consecutive series and the patients were from Emergency Department. The inpatient (validation) cohort was a convenience series and the patients were from medical wards. Their serum samples at the first 24 h after enrollment were collected and subjected to TNC measurement using ELISA. The association between serum TNC level and 28-day all-cause mortality was investigated, and then the predictive value of serum TNC was analyzed. Results: A total of 110 patients with a median age of 64 years (53, 73) were enrolled in the emergency cohort. Compared to the survivors, serum TNC in the non-survivors was significantly higher (467.7 vs. 197.5 ng/ml, p < 0.001). Multivariate logistic regression analysis revealed that the association between serum TNC and 28-day mortality was independent of sepsis or critical illness scores such as SOFA, Acute Physiology and Chronic Health Evaluation (APACHE II), and Simplified Acute Physiology Score (SAPS II), respectively (p < 0.001 for each). The area under receiver operating characteristic curve of serum TNC for predicting mortality was 0.803 (0.717-0.888) (p < 0.001), similar with SOFA 0.808 (0.725-0.891), APACHE II 0.762 (0.667-0.857), and SAPS II 0.779 (0.685-0.872). The optimal cut-off value of serum TNC was 298.2 ng/ml. Kaplan-Meier analysis showed that the survival of patients with serum TNC ≥ 300 ng/ml was significantly worse than that of patients with serum TNC < 300 ng/ml. This result was validated in the inpatient cohort. The sensitivity and specificity of serum TNC ≥ 300 ng/ml for predicting mortality were 74.3 and 74.7% in the emergency cohort, and 63.0 and 70.1% in the inpatient cohort, respectively. Conclusion: Serum TNC was associated with mortality in critically ill patients with multiple organ dysfunction, and would be used as a prognostic tool for predicting mortality in this population.

The Functional Impact of Alternative Splicing and Single Nucleotide Polymorphisms in Rheumatoid Arthritis

Advances in genomics and proteomics aid the identification of genes associated with various diseases. Genome-Wide Association Studies (GWAS) have identified multiple loci as risk alleles for susceptibility to Rheumatoid Arthritis (RA). A bisection of RA risk can be attributed to genetic factors. Over 100 associated genetic loci that encompass immune regulatory factors have been found to be linked with RA. Aberrant Single Nucleotide Polymorphisms (SNPs) and alternative splicing mechanisms in such loci induce RA. These aberrations are viewed as potential therapeutic targets due to their association with a multitude of diseases. This review presents a few imperious genes whose alterations can cause severe bone deformities culminating in RA.

Location, location, location: how the tissue microenvironment affects inflammation in RA

Current treatments for rheumatoid arthritis (RA) do not work well for a large proportion of patients, or at all in some individuals, and cannot cure or prevent this disease. One major obstacle to developing better drugs is a lack of complete understanding of how inflammatory joint disease arises and progresses. Emerging evidence indicates an important role for the tissue microenvironment in the pathogenesis of RA. Each tissue is made up of cells surrounded and supported by a unique extracellular matrix (ECM). These complex molecular networks define tissue architecture and provide environmental signals that programme site-specific cell behaviour. In the synovium, a main site of disease activity in RA, positional and disease stage-specific cellular diversity exist. Improved understanding of the architecture of the synovium from gross anatomy to the single-cell level, in parallel with evidence demonstrating how the synovial ECM is vital for synovial homeostasis and how dysregulated signals from the ECM promote chronic inflammation and tissue destruction in the RA joint, has opened up new ways of thinking about the pathogenesis of RA. These new ideas provide novel therapeutic approaches for patients with difficult-to-treat disease and could also be used in disease prevention.

Tenascin-C Function in Glioma: Immunomodulation and Beyond

First identified in the 1980s, tenascin-C (TNC) is a multi-domain extracellular matrix glycoprotein abundantly expressed during the development of multicellular organisms. TNC level is undetectable in most adult tissues but rapidly and transiently induced by a handful of pro-inflammatory cytokines in a variety of pathological conditions including infection, inflammation, fibrosis, and wound healing. Persistent TNC expression is associated with chronic inflammation and many malignancies, including glioma. By interacting with its receptor integrin and a myriad of other binding partners, TNC elicits context- and cell type-dependent function to regulate cell adhesion, migration, proliferation, and angiogenesis. TNC operates as an endogenous activator of toll-like receptor 4 and promotes inflammatory response by inducing the expression of multiple pro-inflammatory factors in innate immune cells such as microglia and macrophages. In addition, TNC drives macrophage differentiation and polarization predominantly towards an M1-like phenotype. In contrast, TNC shows immunosuppressive function in T cells. In glioma, TNC is expressed by tumor cells and stromal cells; high expression of TNC is correlated with tumor progression and poor prognosis. Besides promoting glioma invasion and angiogenesis, TNC has been found to affect the morphology and function of tumor-associated microglia/macrophages in glioma. Clinically, TNC can serve as a biomarker for tumor progression; and TNC antibodies have been utilized as an adjuvant agent to deliver anti-tumor drugs to target glioma. A better mechanistic understanding of how TNC impacts innate and adaptive immunity during tumorigenesis and tumor progression will open new therapeutic avenues to treat brain tumors and other malignancies.

Tenascin-C in Osteoarthritis and Rheumatoid Arthritis

Tenascin-C (TNC) is a large multimodular glycoprotein of the extracellular matrix that consists of four distinct domains. Emerging evidence suggests that TNC may be involved in the pathogenesis of osteoarthritis (OA) and rheumatoid arthritis (RA). In this review, we summarize the current understanding of the role of TNC in cartilage and in synovial biology, across both OA and RA. TNC is expressed in association with the development of articular cartilage; the expression decreases during maturation of chondrocytes and disappears almost completely in adult articular cartilage. TNC expression is increased in diseased cartilage, synovium, and synovial fluid in OA and RA. In addition, elevated circulating TNC levels have been detected in the blood of RA patients. Thus, TNC could be used as a novel biochemical marker for OA and RA, although it has no specificity as a biochemical marker for these joint disorders. In a post-traumatic OA model of aged joints, TNC deficiency was shown to enhance cartilage degeneration. Treatment with TNC domains results in different, domain-specific effects, which are also dose-dependent. For instance, some TNC fragments including the fibrinogen-like globe domain might function as endogenous inducers of synovitis and cartilage matrix degradation through binding with toll-like receptor-4, while full-length TNC promotes cartilage repair and prevents the development of OA without exacerbating synovitis. The TNC peptide TNIIIA2 also prevents cartilage degeneration without causing synovial inflammation. The clinical significance of TNC effects on cartilage and synovium is unclear and understanding the clinical significance of TNC is not straightforward.

Elevated tenascin-C serum levels in patients with axial spondyloarthritis

This study aimed to examine serum tenascin C (TNC) in different subsets of axial spondyloarthritis (axSpA) patients. Sixty-one patients fulfilling the Assessment of SpondyloArthritis international Society classification criteria for axSpA and 20 healthy subjects (HS) were included in study. Based on imaging, patients were classified as non-radiographic (n=16) and radiographic (n=45) axSpA. TNC serum levels were determined by ELISA. Disease-related factors including the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) and C-reactive protein (CRP) levels were determined. TNC levels were elevated in axSpA patients [535.3 (457.7–677.2) ng/ml] compared to HS [432.1 (329.1–565.9) ng/ml, p=0.007]. Dividing axSpA into radiographic and non-radiographic subsets, the difference in TNC was observed between the radiographic subset and HS [535.3 (434.5–677.2) vs. 432.1 (329.1–565.9) ng/ml, p=0.022]. TNC levels did not correlate with disease activity measures (serum CRP or BASDAI). Nevertheless, the weak correlation of TNC levels with different disease stages (r=0.25, p=0.025) was found, with the highest levels in patients with syndesmophytes. TNC levels are elevated across various subsets of axSpA, and although not related to systemic disease activity, TNC levels might reflect chronic structural spinal changes in axSpA patients. However, its specific role in bone metabolism should be elucidated in further studies.

FC-99 reduces macrophage tenascin-C expression by upregulating miRNA-494 in arthritis

The excessive production of inflammatory mediators by inflammatory cells contributes to the pathogenesis of rheumatoid arthritis. Tenascin-C (TN-C) is expressed in rheumatoid joint, and is associated with levels of inflammatory mediators. FC-99 (N1-[(4-methoxy)methyl]-4-methyl-1,2-Benzenediamine), a novel 1,2-benzenediamine derivative, was previously reported to block the prolonged expression of key rheumatoid arthritis inflammatory cytokines and relieve zymosan-induced joint inflammation. However, the specific mechanism is unknown. This study aimed to examine the effects of FC-99 on TN-C expression and inflammation and investigate its possible molecular mechanism. The results showed that FC-99 treatment reduced the high expression of TN-C in ankle joints of arthritis mice. Besides, FC-99 reduced the increased number of macrophages in arthritis mice, while did not change the number of synovioblasts. Concomitantly, expression of TN-C in synovial fibroblasts exhibited no difference between control and ZIA groups, and was not apparently altered following FC-99 treatment, while FC-99 decreased TN-C expression in macrophages both in vivo and in vitro. Meanwhile, TargetScan and luciferase assays indicated that TN-C was negatively regulated by miR-494. Transfection assay further demonstrated that FC-99 inhibited TN-C by targeting miR-494. Furthermore, the reduction of miR-494 mimic on expression of TN-C was associated with NF-κB pathway. Similarly, the down-regulation of FC-99 on TN-C was considerably decreased when NF-κB pathway was inhibited. These results indicated that FC-99 relieved macrophages inflammation via the miR-494/TN-C/NF-κB pathway, finally leading to the relief of inflammation in arthritis. The findings suggested that FC-99 might be a potential therapeutic candidate for the treatment of rheumatoid arthritis.

Airway Epithelial Cells Generate Pro-inflammatory Tenascin-C and Small Extracellular Vesicles in Response to TLR3 Stimuli and Rhinovirus Infection

Viral infections are a common cause of asthma exacerbations, with human rhinoviruses (RV) the most common trigger. RV signals through a number of different receptors, including toll-like receptor (TLR)3. Tenascin-C (TN-C) is an immunomodulatory extracellular matrix protein present in high quantities in the airway of people with asthma, and expression is also upregulated in nasal lavage fluid in response to RV infection. Respiratory viral infection has been demonstrated to induce the release of small extracellular vesicles (sEV) such as exosomes, whilst exosomal cargo can also be modified in the bronchoalveolar lavage fluid of people with asthma. These sEVs may potentiate airway inflammation and regulate the immune response to infection. This study characterizes the relationship between RV infection of bronchial epithelial cells and the release of TN-C, and the release of sEVs following stimulation with the TLR3 agonist and synthetic viral mimic, poly(I:C), as well as the function of the released protein/vesicles. The BEAS-2B airway epithelial cell line and primary human bronchial epithelial cells (PBECs) from asthmatic and non-asthmatic donors were infected with RV or treated with poly(I:C). TN-C expression, release and localization to sEVs was quantified. TN-C expression was also assessed following intra-nasal challenge of C57BL/6 mice with poly(I:C). BEAS-2B cells and macrophages were subsequently challenged with TN-C, or with sEVs generated from BEAS-2B cells pre-treated with siRNA targeted to TN-C or control. The results revealed that poly(I:C) stimulation induced TN-C release in vivo, whilst both poly(I:C) stimulation and RV infection promoted release in vitro, with elevated TN-C release from PBECs obtained from people with asthma. Poly(I:C) also induced the release of TN-C-rich sEVs from BEAS-2B cells. TN-C, and sEVs from poly(I:C) challenged cells, induced cytokine synthesis in macrophages and BEAS-2B cells, whilst sEVs from control cells did not. Moreover, sEVs with ~75% reduced TN-C content did not alter the capacity of sEVs to induce inflammation. This study identifies two novel components of the inflammatory pathway that regulates the immune response following RV infection and TLR3 stimulation, highlighting TN-C release and pro-inflammatory sEVs in the airway as relevant to the biology of virally induced exacerbations of asthma.

Monocyte and haematopoietic progenitor reprogramming as common mechanism underlying chronic inflammatory and cardiovascular diseases

A large number of cardiovascular events are not prevented by current therapeutic regimens. In search for additional, innovative strategies, immune cells have been recognized as key players contributing to atherosclerotic plaque progression and destabilization. Particularly the role of innate immune cells is of major interest, following the recent paradigm shift that innate immunity, long considered to be incapable of learning, does exhibit immunological memory mediated via epigenetic reprogramming. Compelling evidence shows that atherosclerotic risk factors promote immune cell migration by pre-activation of circulating innate immune cells. Innate immune cell activation via metabolic and epigenetic reprogramming perpetuates a systemic low-grade inflammatory state in cardiovascular disease (CVD) that is also common in other chronic inflammatory disorders. This opens a new therapeutic area in which metabolic or epigenetic modulation of innate immune cells may result in decreased systemic chronic inflammation, alleviating CVD, and its co-morbidities.

Internal Affairs: Tenascin-C as a Clinically Relevant, Endogenous Driver of Innate Immunity

To protect against danger, the innate immune system must promptly and accurately sense alarm signals, and mount an appropriate response to restore homeostasis. One endogenous trigger of immunity is tenascin-C, a large hexameric protein of the extracellular matrix. Upregulated upon tissue injury and cellular stress, tenascin-C is expressed during inflammation and tissue remodeling, where it influences cellular behavior by interacting with a multitude of molecular targets, including other matrix components, cell surface proteins, and growth factors. Here, we discuss how these interactions confer upon tenascin-C distinct immunomodulatory capabilities that make this matrix molecule necessary for efficient tissue repair. We also highlight in vivo studies that provide insight into the consequences of misregulated tenascin-C expression on inflammation and fibrosis during a wide range of inflammatory diseases. Finally, we examine how its unique expression pattern and inflammatory actions make tenascin-C a viable target for clinical exploitation in both diagnostic and therapeutic arenas.

Tenascin-C Prevents Articular Cartilage Degeneration in Murine Osteoarthritis Models

Objective The objective of this study was to determine whether intra-articular injections of tenascin-C (TNC) could prevent cartilage damage in murine models of osteoarthritis (OA). Design Fluorescently labeled TNC was injected into knee joints and its distribution was examined at 1 day, 4 days, 1 week, 2 weeks, and 4 weeks postinjection. To investigate the effects of TNC on cartilage degeneration after surgery to knee joints, articular spaces were filled with 100 μg/mL (group I), 10 μg/mL (group II) of TNC solution, or control (group III). TNC solution of 10 μg/mL was additionally injected twice after 3 weeks (group IV) or weekly after 1 week, 2 weeks, and 3 weeks (group V). Joint tissues were histologically assessed using the Mankin score and the modified Chambers system at 2 to 8 weeks after surgery. Results Exogenous TNC was maintained in the cartilage and synovium for 1 week after administration. Histological scores in groups I and II were better than scores in group III at 4 and 6 weeks, but progressive cartilage damage was seen in all groups 8 weeks postoperatively. Sequential TNC injections (groups IV and V) showed significantly better Mankin score than single injection (group II) at 8 weeks. Conclusion TNC administered exogenously remained in the cartilage of knee joints for 1 week, and could decelerate articular cartilage degeneration in murine models of OA. We also showed that sequential administration of TNC was more effective than a single injection. TNC could be an important molecule for prevention of articular cartilage damage.

Synovial fibroblasts in 2017

Stromal cells like synovial fibroblasts gained great interest over the years, since it has become clear that they strongly influence their environment and neighbouring cells. The current review describes the role of synovial fibroblasts as cells of the innate immune system and expands on their involvement in inflammation and cartilage destruction in rheumatoid arthritis (RA). Furthermore, epigenetic changes in RA synovial fibroblasts and studies that focused on the identification of different subsets of synovial fibroblasts are discussed.

The role of α9β1 integrin and its ligands in the development of autoimmune diseases

Adhesion of cells to extracellular matrix proteins through integrins expressed on the cell surface is important for cell adhesion/motility, survival, and differentiation. Recently, α9β1 integrin was reported to be important for the development of autoimmune diseases including rheumatoid arthritis, multiple sclerosis, and their murine models. In addition, ligands for α9β1 integrin, such as osteopontin and tenascin-C, are well established as key regulators of autoimmune diseases. Therefore, this review focused on the role of interactions between α9β1 integrin and its ligands in the development of autoimmune diseases.

Role of tenascin-C in articular cartilage

Tenascin-C (TN-C) is a glycoprotein component of the extracellular matrix (ECM). TN-C consists of four distinct domains, including the tenascin assembly domain, epidermal growth factor-like repeats, fibronectin type III-like repeats, and the fibrinogen-like globe (FBG) domain. This review summarizes the role of TN-C in articular cartilage. Expression of TN-C is associated with the development of articular cartilage but markedly decreases during maturation of chondrocytes and disappears almost completely in adult articular cartilage. Increased expression of TN-C has been found at diseased cartilage and synovial sites in osteoarthritis (OA) and rheumatoid arthritis (RA). TN-C is increased in the synovial fluid in patients with OA and RA. In addition, serum TN-C is elevated in RA patients. TN-C could be a useful biochemical marker for joint disease. The addition of TN-C results in different effects among TN-C domains. TN-C fragments might be endogenous inducers of cartilage matrix degradation; however, full-length TN-C could promote cartilage repair and prevent cartilage degeneration. The deficiency of TN-C enhanced cartilage degeneration in the spontaneous OA in aged joints and surgical OA model. The clinical significance of TN-C effects on cartilage is not straightforward.

A specific anti-citrullinated protein antibody profile identifies a group of rheumatoid arthritis patients with a toll-like receptor 4-mediated disease

BACKGROUND

Increased expression of toll-like receptor 4 (TLR4) and its endogenous ligands, is characteristic of rheumatoid arthritis (RA) synovitis. In this study, we evaluated how these TLR4 ligands may drive pathogenic processes and whether the fine profiling of anti-citrullinated protein antibodies (ACPA) based on their target specificity might provide a simple means to predict therapeutic benefit when neutralizing TLR4 in this disease.

METHODS

The capacity of RA synovial fluids (RASF) to stimulate cytokine production in monocytes from patients with RA was analyzed by ELISA. The presence of TLR4 activators in RASF was determined by measuring the levels of ACPA, ACPA subtypes with reactivity to specific citrullinated peptides and other TLR4 ligands. Neutralization of TLR4 signaling was investigated using NI-0101, a therapeutic antibody that targets TLR4.

RESULTS

RASF exhibited a heterogeneous capacity to induce production of proinflammatory cytokines by monocytes isolated from patients with RA. Such cytokine responses were significantly modified by TLR4 blockade achieved using NI-0101. The analysis of the content of RASF and matched sera demonstrated that ACPA fine specificities in patient samples predict cellular response to anti-TLR4 exposure in vitro.

CONCLUSION

TLR4 represents a possible therapeutic target in RA. Our study demonstrates that TLR4 inhibition in an ex vivo model of RA pathogenesis can significantly modulate cytokine release and does so in specific subgroups of RA patient-derived samples. It also suggests that ACPA fine profiling has the potential to identify RA patients with a predominantly TLR4-driven pathotype that could be used to predict preferential response to TLR4 antagonism.

Identification of an immunodominant peptide from citrullinated tenascin-C as a major target for autoantibodies in rheumatoid arthritis

Objectives

We investigated whether citrullinated tenascin-C (cTNC), an extracellular matrix protein expressed at high levels in the joints of patients with rheumatoid arthritis (RA), is a target for the autoantibodies in RA.

Methods

Citrullinated sites were mapped by mass spectrometry in the fibrinogen-like globe (FBG) domain of tenascin-C treated with peptidylarginine deiminases (PAD) 2 and 4. Antibodies to cyclic peptides containing citrullinated sites were screened in sera from patients with RA by ELISA. Potential cross-reactivity with well-established anticitrullinated protein antibody (ACPA) epitopes was tested by inhibition assays. The autoantibody response to one immunodominant cTNC peptide was then analysed in 101 pre-RA sera (median 7 years before onset) and two large independent RA cohorts.

Results

Nine arginine residues within FBG were citrullinated by PAD2 and PAD4. Two immunodominant peptides cTNC1 (VFLRRKNG-cit-ENFYQNW) and cTNC5 (EHSIQFAEMKL-cit-PSNF-cit-NLEG-cit-cit-KR) were identified. Antibodies to both showed limited cross-reactivity with ACPA epitopes from α-enolase, vimentin and fibrinogen, and no reactivity with citrullinated fibrinogen peptides sharing sequence homology with FBG. cTNC5 antibodies were detected in 18% of pre-RA sera, and in 47% of 1985 Swedish patients with RA and 51% of 287 North American patients with RA. The specificity was 98% compared with 160 healthy controls and 330 patients with osteoarthritis.

Conclusions

There are multiple citrullination sites in the FBG domain of tenascin-C. Among these, one epitope is recognised by autoantibodies that are detected years before disease onset, and which may serve as a useful biomarker to identify ACPA-positive patients with high sensitivity and specificity in established disease.

Tenascin-C Levels, A Toll-like Receptor 4 Ligand, in Enthesitis-related Arthritis Category of Juvenile Idiopathic Arthritis: A Cross-sectional and Longitudinal Study

OBJECTIVE

Monocytes of children with enthesitis-related arthritis (ERA) show Toll-like receptor 4 (TLR4) overexpression. Tenascin-C (TNC) is an extracellular matrix glycoprotein and acts as an endogenous TLR4 ligand. Thus, we studied the serum and synovial fluid (SF) levels of TNC in children with ERA.

METHODS

TNC was measured in the serum of 80 children with ERA satisfying the International League of Associations for Rheumatology criteria. Fifteen children were followed up while being treated with regular nonsteroidal antiinflammatory drug (NSAID) therapy and levels were reassessed at 3 months. Seventeen paired serum-SF samples and 25 healthy control serum samples were also analyzed. Disease activity was assessed by physician's global assessment (PGA), early morning stiffness (EMS), tender (TJC) and swollen joint counts (SJC), enthesitis score, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP).

RESULTS

The mean serum TNC level in patients with active disease (67.1 ± 44.9 ng/ml) was significantly higher than in those with inactive disease (40.6 ± 36.7 ng/ml, p = 0.01) and healthy controls (21 ± 15.2 ng/ml, p < 0.001). Levels of TNC were higher in HLA-B27-positive (58.4 ng/ml) versus -negative disease (20.4 ng/ml, p = 0.01). TNC levels correlated moderately with disease activity: PGA r = 0.4, EMS r = 0.34, TJC r = 0.4, SJC r = 0.46, ESR r = 0.42, and CRP r = 0.32. In receiver-operation characteristic analysis for active versus inactive diseases, TNC [area under the curve (AUC) = 0.754] was equivalent to ESR (AUC = 0.787) and CRP (AUC = 0.789). Regular NSAID therapy led to a significant fall in serum TNC levels at 3 months (p = 0.0003). The SF TNC level was 17.39 ng/ml, significantly lower than the paired serum values (p = 0.01).

CONCLUSION

Serum TNC levels are significantly raised and correlate with various clinical and laboratory variables of disease activity in children with ERA. Regular NSAID therapy reduces the TNC levels, probably related to controlling disease activity.

α9β1 integrin acts as a critical intrinsic regulator of human rheumatoid arthritis

OBJECTIVE

The role of the joint tissue microenvironment in the pathogenesis of human RA has recently attracted much attention. The present study investigated the roles of α9β1 integrin and its ligands in synovial specimens of human RA patients in generating the unique human arthritic tissue microenvironment.

METHODS

Synovial fibroblasts and macrophages were isolated from the synovial tissue of patients with RA or OA. The expression of α9β1 integrin was analysed using FACS with multicolour staining. The production of MMPs and proinflammatory cytokines was analysed in cultures of synovial fibroblasts and macrophages with α9β1 integrin ligands.

RESULTS

Synovial fibroblasts and macrophages derived from arthritic joints spontaneously secreted tenascin-C and osteopontin. Synovial fibroblasts and macrophages obtained from patients with RA expressed α9β1 integrins, a common receptor for osteopontin and tenascin-C. In the synovial fibroblasts of RA, the amount of tenascin-C protein produced was much greater than that of osteopontin in synovial fibroblasts of RA. Importantly, autocrine and paracrine interactions of α9β1 integrin and tenascin-C induced the expression of MMPs and IL-6 in synovial fibroblasts, as well as TNF-α and IL-1β in synovial macrophages.

CONCLUSION

These findings indicate that autocrine and paracrine interaction of α9β1 integrin and tenascin-C in the joint tissue microenvironment contributes to the pathogenesis of RA. Therefore α9β1 integrin may become a potential therapeutic target for RA.

Aberrant and alternative splicing in skeletal system disease

The main function of skeletal system is to support the body and help movement. A variety of factors can lead to skeletal system disease, including age, exercise, and of course genetic makeup and expression. Pre-mRNA splicing plays a crucial role in gene expression, by creating multiple protein variants with different biological functions. The recent studies show that several skeletal system diseases are related to pre-mRNA splicing. This review focuses on the relationship between pre-mRNA splicing and skeletal system disease. On the one hand, splice site mutation that leads to aberrant splicing often causes genetic skeletal system disease, like COL1A1, SEDL and LRP5. On the other hand, alternative splicing without genomic mutation may generate some marker protein isoforms, for example, FN, VEGF and CD44. Therefore, understanding the relationship between pre-mRNA splicing and skeletal system disease will aid in uncovering the mechanism of disease and contribute to the future development of gene therapy.

Tenascin-C levels in synovial fluid are elevated after injury to the human and canine joint and correlate with markers of inflammation and matrix degradation

OBJECTIVE

We have previously shown the capacity of tenascin-C (TN-C) to induce inflammatory mediators and matrix degradation in vitro in human articular cartilage. The objective of the present study was to follow TN-C release into knee synovial fluid after acute joint injury or in joint disease, and to correlate TN-C levels with markers of cartilage matrix degradation and inflammation.

METHOD

Human knee synovial fluid samples (n = 164) were from a cross-sectional convenience cohort. Diagnostic groups were knee healthy reference, knee anterior cruciate ligament rupture, with or without concomitant meniscus lesions, isolated knee meniscus injury, acute inflammatory arthritis (AIA) and knee osteoarthritis (OA). TN-C was measured in synovial fluid samples using an enzyme-linked immunosorbent assay (ELISA) and results correlated to other cartilage markers. TN-C release was also monitored in joints of dogs that underwent knee instability surgery.

RESULTS

Statistically significantly higher levels of TN-C compared to reference subjects were observed in the joint fluid of all human disease groups and in the dogs that underwent knee instability surgery. Statistically significant correlations were observed between the TN-C levels in the synovial fluid of the human patients and the levels of aggrecanase-dependent Ala-Arg-Gly-aggrecan (ARG-aggrecan) fragments and matrix metalloproteinases 1 and 3.

CONCLUSIONS

We find highly elevated levels of TN-C in human knee joints after injury, AIA or OA that correlated with markers of cartilage degradation and inflammation. TN-C in synovial fluid may serve dual roles as a marker of joint damage and a stimulant of further joint degradation.

Raised circulating tenascin-C in rheumatoid arthritis

Introduction

The aim of this study was to examine whether circulating levels of the pro-inflammatory glycoprotein tenascin-C (TNC) are elevated in musculoskeletal disorders including rheumatoid arthritis (RA) and to assess in RA whether levels are related to clinical disease status and/or patient response to treatment.

Methods

TNC in serum or plasma was quantified by ELISA. Samples from 4 cohorts of RA patients were examined and compared to normal human subjects and to patients with other inflammatory diseases.

Results

Circulating TNC levels were significantly raised in patients with RA, as well as patients with systemic lupus erythematosus, idiopathic inflammatory myositis, psoriatic arthritis and ankylosing spondylitis, whilst patients with Sjogren's syndrome displayed levels similar to healthy controls. The highest levels of TNC were observed in RA patients with late stage disease. In early disease TNC levels correlated positively with ultrasound determined erosion scores. Treatment of early RA patients with infliximab plus methotrexate (MTX) resulted in a transient decrease in circulating TNC over the first year of therapy. In contrast, TNC levels increased over time in RA patients receiving MTX alone. In patients treated with infliximab plus MTX, baseline TNC levels significantly correlated with tender joint counts (TJC) at 18 and 54 weeks after initiation of infliximab therapy.

Conclusions

Raised circulating TNC levels are detected in specific inflammatory diseases. Levels are especially high in RA where they may act as a biomarker of bone erosion and a predictor of the effect of infliximab on RA patient joint pain.

Syndecans in cartilage breakdown and synovial inflammation

Syndecans are transmembrane heparan sulphate proteoglycans (HSPGs) that have gained increasing interest as regulators of a variety of tissue responses, including cartilage development and remodelling. These proteoglycans are composed of a core protein to which extracellular glycosaminoglycan (GAG) chains are attached. Through these GAG chains, syndecans can interact with a variety of extracellular matrix molecules and bind to a number of soluble mediators including morphogens, growth factors, chemokines and cytokines. The structure and post-translational modification of syndecan GAG chains seem to differ not only from cell to cell, but also during different stages of cellular differentiation, leading to a complexity of syndecan function that is unique among membrane-bound HSPGs. Unlike other membrane-bound HSPGs, syndecans contain intracellular signalling motifs that can initiate signalling mainly through protein kinase C. This Review summarizes our knowledge of the biology of syndecans and the mechanisms by which binding of molecules to syndecans exert different biological effects, particularly in the joints. On the basis of the structural and functional peculiarities of syndecans, we discuss the regulation of syndecans and their roles in the developing joint as well as during degenerative and inflammatory cartilage remodelling as understood from expression studies and functional analyses involving syndecan-deficient mice.

Toll-like receptor--a potent driving force behind rheumatoid arthritis

Toll like receptor (TLR), one of the key functions of innate immune system, can recognize not only exogenous pathogen-associated molecular patterns, namely PAMPs, but also endogenous molecules created upon tissue injury, sterile inflammation and degeneration. Endogenous TLR ligands are called as damage-associated molecular patters (DAMPs), including endogenous molecules released by activated and necrotic cells, and extracellular matrix molecules. DAMPs are also known as alarmins. TLR research has brought about new insights in the rheumatic diseases. Previous reports suggest that TLRs and the signal pathways intensively contribute to the pathogenesis of rheumatoid arthritis (RA) and other arthritic conditions with interaction of various TLR ligands. Accumulated knowledge of TLR system is summarized to overlook TLRs and the signaling pathway in arthritis conditions, with special reference to RA.

Tenascin-C fragments are endogenous inducers of cartilage matrix degradation

Cartilage destruction is a hallmark of osteoarthritis (OA) and is characterized by increased protease activity resulting in the degradation of critical extracellular matrix (ECM) proteins essential for maintaining cartilage integrity. Tenascin-C (TN-C) is an ECM glycoprotein, and its expression is upregulated in OA cartilage. We aimed to investigate the presence of TN-C fragments in arthritic cartilage and establish whether they promote cartilage degradation. Expression of TN-C and its fragments was evaluated in cartilage from subjects undergoing joint replacement surgery for OA and RA compared with normal subjects by western blotting. The localization of TN-C in arthritic cartilage was also established by immunohistochemistry. Recombinant TN-C fragments were then tested to evaluate which regions of TN-C are responsible for cartilage-degrading activity in an ex vivo cartilage explant assay measuring glycosaminoglycan (GAG) release, aggrecanase and matrix metalloproteinase (MMP) activity. We found that specific TN-C fragments are highly upregulated in arthritic cartilage. Recombinant TN-C fragments containing the same regions as those identified from OA cartilage mediate cartilage degradation by the induction of aggrecanase activity. TN-C fragments mapping to the EGF-L and FN type III domains 3–8 of TN-C had the highest levels of aggrecan-degrading ability that was not observed either with full-length TN-C or with other domains of TN-C. TN-C fragments represent a novel mechanism for cartilage degradation in arthritis and may present new therapeutic targets for the inhibition of cartilage degradation.

Tenascin-C induces inflammatory mediators and matrix degradation in osteoarthritic cartilage

Background

Tenascin-C (TN-C) is an extracellular matrix glycoprotein that is involved in tissue injury and repair processes. We analyzed TN-C expression in normal and osteoarthritic (OA) human cartilage, and evaluated its capacity to induce inflammatory and catabolic mediators in chondrocytes in vitro. The effect of TN-C on proteoglycan loss from articular cartilage in culture was also assessed.

Methods

TN-C in culture media, cartilage extracts, and synovial fluid of human and animal joints was quantified using a sandwich ELISA and/or analyzed by Western immunoblotting. mRNA expression of TN-C and aggrecanases were analyzed by Taqman assays. Human and bovine primary chondrocytes and/or explant culture systems were utilized to study TN-C induced inflammatory or catabolic mediators and proteoglycan loss. Total proteoglycan and aggrecanase -generated ARG-aggrecan fragments were quantified in human and rat synovial fluids by ELISA.

Results

TN-C protein and mRNA expression were significantly upregulated in OA cartilage with a concomitant elevation of TN-C levels in the synovial fluid of OA patients. IL-1 enhanced TN-C expression in articular cartilage. Addition of TN-C induced IL-6, PGE₂, and nitrate release and upregulated ADAMTS4 mRNA in cultured primary human and bovine chondrocytes. TN-C treatment resulted in an increased loss of proteoglycan from cartilage explants in culture. A correlation was observed between TN-C and aggrecanase generated ARG-aggrecan fragment levels in the synovial fluid of human OA joints and in the lavage of rat joints that underwent surgical induction of OA.

Conclusions

TN-C expression in the knee cartilage and TN-C levels measured in the synovial fluid are significantly enhanced in OA patients. Our findings suggest that the elevated levels of TN-C could induce inflammatory mediators and promote matrix degradation in OA joints.

DAMPening inflammation by modulating TLR signalling

Damage-associated molecular patterns (DAMPs) include endogenous intracellular molecules released by activated or necrotic cells and extracellular matrix (ECM) molecules that are upregulated upon injury or degraded following tissue damage. DAMPs are vital danger signals that alert our immune system to tissue damage upon both infectious and sterile insult. DAMP activation of Toll-like receptors (TLRs) induces inflammatory gene expression to mediate tissue repair. However, DAMPs have also been implicated in diseases where excessive inflammation plays a key role in pathogenesis, including rheumatoid arthritis (RA), cancer, and atherosclerosis. TLR activation by DAMPs may initiate positive feedback loops where increasing tissue damage perpetuates pro-inflammatory responses leading to chronic inflammation. Here we explore the current knowledge about distinct signalling cascades resulting from self TLR activation. We also discuss the involvement of endogenous TLR activators in disease and highlight how specifically targeting DAMPs may yield therapies that do not globally suppress the immune system.

Alternative splicing in multiple sclerosis and other autoimmune diseases

Alternative splicing is a general mechanism for regulating gene expression that affects the RNA products of more than 90% of human genes. Not surprisingly, alternative splicing is observed among gene products of metazoan immune systems, which have evolved to efficiently recognize pathogens and discriminate between "self" and "non-self", and thus need to be both diverse and flexible. In this review we focus on the specific interface between alternative splicing and autoimmune diseases, which result from a malfunctioning of the immune system and are characterized by the inappropriate reaction to self-antigens. Despite the widespread recognition of alternative splicing as one of the major regulators of gene expression, the connections between alternative splicing and autoimmunity have not been apparent. We summarize recent findings connecting splicing and autoimmune disease, and attempt to find common patterns of splicing regulation that may advance our understanding of autoimmune diseases and open new avenues for therapy.

Transcriptional regulation of the endogenous danger signal tenascin-C: a novel autocrine loop in inflammation

Inappropriate expression of proinflammatory mediators underpins the pathogenesis of autoimmune disease and tumor metastasis. The extracellular matrix glycoprotein tenascin-C is an endogenous activator of innate immunity that promotes the synthesis of inflammatory cytokines via activation of TLR4. Little tenascin-C is observed in most healthy adult tissues, but expression is specifically upregulated at sites of inflammation. Moreover, high levels of tenascin-C are associated with chronic inflammation and found in the tumor stroma. In this study, we show that the expression of tenascin-C is induced in immune myeloid cells activated by a variety of inflammatory stimuli, including specific TLR ligands. Its synthesis is transcriptionally regulated and requires the specific activation of AKT/PI3K and NF-kappaB signaling pathways. Using a bioinformatic approach, we identified a large number of conserved noncoding regions throughout the tenascin-C genomic locus that may contribute to its transcriptional regulation during inflammation. We also demonstrate that tenascin-C expression is transient during acute inflammation. In contrast, persistently high levels of expression occur in the inflamed synovium of joints from rheumatoid arthritis patients. Thus, misregulated expression of this endogenous danger signal may promote an autocrine loop of inflammation and contribute to the persistence of inflammation in autoimmune diseases or to tumor egress and invasion during metastasis.

Tenascin-C is an endogenous activator of Toll-like receptor 4 that is essential for maintaining inflammation in arthritic joint disease

Although there have been major advances in the treatment of rheumatoid arthritis with the advent of biological agents, the mechanisms that drive cytokine production and sustain disease chronicity remain unknown. Tenascin-C (encoded by Tnc) is an extracellular matrix glycoprotein specifically expressed at areas of inflammation and tissue damage in inflamed rheumatoid joints. Here we show that mice that do not express tenascin-C show rapid resolution of acute joint inflammation and are protected from erosive arthritis. Intra-articular injection of tenascin-C promotes joint inflammation in vivo in mice, and addition of exogenous tenascin-C induces cytokine synthesis in explant cultures from inflamed synovia of individuals with rheumatoid arthritis. Moreover, in human macrophages and fibroblasts from synovia of individuals with rheumatoid arthritis, tenascin-C induces synthesis of proinflammatory cytokines via activation of Toll-like receptor 4 (TLR4). Thus, we have identified tenascin-C as a novel endogenous activator of TLR4-mediated immunity that mediates persistent synovial inflammation and tissue destruction in arthritic joint disease.

Synergistic effects of cyclic strain and Th1-like cytokines on tenascin-C production by rheumatic aortic valve interstitial cells

Tenascin-C (TN-C) is a key component of extracellular matrix (ECM) and its expression process is poorly understood during rheumatic heart valvular disease (RHVD). In this study, we found that interferon (IFN)-gamma, tumour necrosis factor (TNF)-alpha and TN-C concentrations in patients with RHVD were significantly higher than in normal controls. More IFN-gamma receptors and TNF receptors were found being expressed on rheumatic aortic valves interstitial cells than on non-rheumatic ones and their expression was patients' sera dependent. Antibodies neutralizing IFN-gamma or TNF-alpha could attenuate patients' sera-induced TN-C transcription by isolated rheumatic aortic valves interstitial cells. By application with different protein kinase inhibitors, we found that combined with cyclic strain, TNF-alpha and IFN-gamma induced TN-C transcription through the RhoA/ROCK signalling pathway. At the same time, p38 mitogen-activated protein kinase was involved in TNF-alpha and IFN-gamma induced TN-C transcription. TNF-alpha also increased TN-C mRNA level by additional PKC and ERK 1/2 activation. Our finding revealed a new insight into ECM remodelling during RHVD pathogenesis and new mechanisms involved in the clinical anti-IFN-gamma and anti-TNF-alpha therapy.

Changes in biochemical markers and prediction of effectiveness of intra-articular hyaluronan in patients with knee osteoarthritis

OBJECTIVE

Intra-articular injection of hyaluronan (HA) is frequently used to treat knee osteoarthritis (OA). We studied whether HA injections induced significant changes in levels of biochemical markers in synovial fluid (SF). In addition, we investigated the possibility of predicting the effectiveness of HA based on these biochemical markers.

METHODS

Twenty-eight patients with knee OA underwent five weekly intra-articular injections of HA. Knee pain was measured on visual analog scale (VAS) before and after the five injections. Levels of biochemical markers, including chondroitin 6-sulfate (C6S), chondroitin 4-sulfate (C4S), keratan sulfate (KS), and tenascin-C (TN-C), were determined before and after the five injections. Correlations between the biochemical markers before HA injection and the improvement of VAS after the five injections were evaluated.

RESULTS

After HA injections, levels of C6S, C4S, and KS decreased significantly. Inverse correlations were observed between the levels of TN-C and C4S before HA injection and improvement of VAS after the five injections. In contrast, no significant correlation was seen between levels of C6S and KS before injections and improvement of VAS after the five injections.

CONCLUSION

The reduction in C6S, C4S, and KS levels after HA injections reflects that HA could help maintain normal cartilage metabolism. Our findings suggest that HA injections are effective in patients whose knees contain low levels of TN-C and C4S, reflecting an early stage of OA and limited synovitis.